Field of the Invention
The present invention relates to optics. More specifically, the present invention relates to systems and methods for directing and correcting high-power beams of electromagnetic energy.
Description of the Related Art
Directed energy weapons and specifically high-energy laser (HEL) weapons are being considered for variety of military applications with respect to a variety of platforms, e.g., spaceborne, airborne and land based systems to name a few. These weapons generally involve the use of the laser or other source of a high-power beam to track and destroy a target. To achieve mission objectives, directed energy weapons must be accurately steered and optimally focused. Steering involves line-of-sight control and focusing, with respect to HEL weapons, involves wavefront error correction. Currently, wavefront error correction is typically achieved using adaptive optics. The current state of the art in laser beam control adaptive optics requires placing one or more deformable mirrors within the highest intensity portion of the beam path. The conventional deformable mirror is typically a large element with a thin face sheet and a number of piezoelectric actuators. Actuators are located behind the face sheet and are electrically driven to push and pull on the surface thereof to effect the deformation required to correct wavefront errors in an outgoing beam. The size of the active region of the deformable mirror must accommodate the full size of the high power laser beam in the high power Coudxc3xa9 path prior to expansion via an output telescope.
In addition, one or more fast steering mirrors may be used to correct for tilt and direct the line-of-sight. A course gimbal may be employed to correct for line-of-sight errors as well. A plurality of wavefront sensors are typically employed along with an aperture sharing element (ASE). The ASE allows a single shared aperture to be advantageously used for both the low power sensors and the high power output laser beam, ensuring that the path through the atmosphere taken by the high power beam is the same as that taken by the wavefront sensor and that the correction applied to the shared atmospheric path is optimal for the high-power beam.
Unfortunately, the use of delicate optical devices in the path of a high-power beam is problematic. This is due to the fact that the high-power beam will heat and distort the optical element unless the element is actively cooled or has a coating with a very low optical absorption coefficient. The most durable coatings require a high temperature application process. Deformable mirrors are typically coated after the face sheet is bonded to the actuators, which limits the maximum temperature to which the deformable mirror assembly may be exposed without degrading the bond. Therefore, coatings may need to be applied at lower than optimal temperature using more complex coating processes, thereby reducing durability and/or increasing manufacturing cost.
In addition, conventional adaptive optics systems using deformable mirrors are limited in performance. Conventional deformable mirrors systems are limited with respect to the speed at which the mirror drive signals are computed and the reaction speed of the deformable mirror mechanism to correct for aberrations. There is also a limitation with respect to the number actuators that can be used. The number of actuators that may be used determines the resolution or xe2x80x9corderxe2x80x9d of the mirror. The stroke of the conventional deformable mirror is limited. xe2x80x9cStrokexe2x80x9d relates to the amount of mirror surface deflection that may be achieved before either the piezoelectric actuators exceed their dynamic range or the face sheet begins to fail. Further, a conventional continuous face sheet deformable mirror cannot correct for a pathology in the spatial phase pattern, such as a branch point or an abrupt phase discontinuity. A branch point is a xe2x80x9csingularityxe2x80x9d in a deeply scintillated phase pattern caused by atmospheric turbulence over a long propagation path in which the phase monotonically increases around a zero amplitude point like a corkscrew, thereby requiring an abrupt 2xcfx80 phase correction within the spatial phase pattern. Abrupt phase discontinuities may be caused by the optical discontinuities between segments of a multi-segment primary mirror.
In U.S. Pat. No. 5,694,408, issued Dec. 2, 1997, (the teachings of which are incorporated herein by reference), Bott, Rice, and Zediker appear to disclose a scheme which allows the deformable element to be placed in the low intensity region between a master oscillator and an array of fiber power amplifiers. The approach is to pre-distort the phase of the oscillator beamlets after separation in a distribution network and before injection into the fiber amplifier array, such that the pre-distortion corrects both the piston error between the individual fibers and optical aberrations in the atmosphere. However, this scheme is practical only with a coherently-combined array of single-mode fiber amplifiers, as each fiber channel is correctable in piston only, not high order. Also, this scheme is not applicable to multi-mode laser media such as large core fiber amplifiers or bulk media lasers as contemplated for weapon class HEL devices and may not be scaleable to high power levels due to random, high frequency phase noise caused by pump-induced temperature fluctuations within the fibers.
In U.S. Pat. No. 5,090,795, issued Feb. 25, 1992, the teachings of which are incorporated herein by reference, O""Meara and Valley appear to disclose several related schemes for using a liquid crystal light valve (LCLV) in a self-correcting adaptive optics system. This approach, however, places the LCLV in the high power beam path and is therefore limited by the damage susceptibility of the liquid crystal material.
Accordingly, a need remains in the art for a fast, large-stroke, high spatial bandwidth or high order system or method for effecting wavefront correction of a high-power beam. Ideally, such a wavefront correction system or method would operate moduli 2xcfx80, i.e., would accommodate an instantaneous 2xcfx80 phase jump anywhere within the phase pattern.
The need in the art is addressed by the beam control system and method of the present invention. The invention is adapted for use with a system for illuminating a target with a first beam of electromagnetic energy. Several embodiments are disclosed herein. In a preferred embodiment, the inventive system includes a first mechanism for receiving a first beam of electromagnetic energy; a second mechanism for detecting aberrations in the first beam; a third mechanism responsive to the second mechanism for generating a second beam that is at least partially compensated with respect to the aberrations detected; and a fourth mechanism for amplifying the second beam to provide an output beam.
In a more specific embodiment, the inventive system includes a first mechanism for receiving a target return comprising a reflection of the first beam from the target. A second mechanism is included for correcting for aberrations in the wavefront of the target return. A third mechanism is included for ascertaining the correction applied by the second mechanism to the target return. A fourth mechanism applies the correction to a third beam, with the third beam ultimately being an output beam. In the more specific embodiment, the first beam of electromagnetic energy is optical energy and the first mechanism is a telescope. The fourth mechanism includes a first phase conjugate mirror adapted to conjugate electromagnetic energy output by the third mechanism and a second phase conjugate mirror adapted to conjugate the output of the first phase conjugate mirror. The fourth mechanism further includes an amplifier for boosting the signal output by the second phase conjugate mirror to provide the output beam.
In a preferred embodiment, an outcoupling element is disposed between the first mechanism and the second mechanism. In the best mode, the outcoupling element is an aperture sharing element (ASE) and the second mechanism is an optical phased array. A wavefront error sensor is provided to receive a signal output by the optical phased array and provide a wavefront error signal in response thereto. A processor is included and programmed to respond to the wavefront error signal and provide a correction signal to the optical phased array in response thereto. The third mechanism is implemented with a master oscillator adapted to provide a low-power reference beam. The reference beam illuminates the optical phased array and provides a target-path wavefront error corrected signal in response thereto. In this best-mode embodiment, the oscillator beam does not produce a correction detection signal, but rather produces the reference signal that is amplified to generate the actual high-power beam. This signal illuminates the back of the aperture sharing element and back reflects off the front surface of the ASE. This signal, in turn, is conjugated by the first phase conjugate mirror and transmitted through the ASE to the second phase conjugate mirror. The second phase conjugate mirror conjugates the transmitted signal thus canceling the effect of the first phase conjugation process. This signal is then amplified and front reflected off the front surface of the ASE to provide the output beam to the telescope and beam director, where it is directed to the target. As the front and back reflections off the front surface of the ASE are phase conjugates of one another, any reflective distortion due to this element is removed. Refractive distortions in the ASE, laser amplifiers, and other optical elements are also removed in this embodiment via the wavefront reversal properties of the first and second phase conjugate mirrors. Consequently, the output beam is compensated for the optics of the system and includes a correction for the atmosphere provided by the optical phased array.
The invention uses the wavefront reversal property of nonlinear optical phase conjugation to permit incorporation of a photonic spatial light modulator, such as a liquid crystal optical phased array (OPA) or a micro electro-mechanical system within the low power legs of the beam control system, thereby affording the advantages of the OPA without the power limitations.